The sheet turning process in printing machines with a perfecting process starts in the perfecting mode--as known in the arts--with the separation of the sheet front edge from the surface of the cylinder preceding the perfecting drum. A lifting mechanism in the cylinder lifts the front edge of the sheet over the prongs of a guide blade which strip the sheet from the cylinder and guide the sheet with pneumatic support along the cylinder contour until the rear end of the sheet is grasped by a sheet holding element (sucker system). Subsequently, the sheet undergoes return of its motion. The pneumatic guiding force for the sheet is produced by a negative pressure and tangential force created by a blow-suction effect. Such guide blades are known from DE 44 24 964 A1 and DE 196 35 388 A1.
With known devices, the front section of the sheet to be turned glides relatively evenly over the guiding surface of the guide blade. However, sheet fluttering can occur when the sheet approaches the return motion. The front part of the sheet, especially the sheet corners can flutter heavily and thus can negatively influencing the sheet guiding and the printing image (smearing). The reason for sheet fluttering in this relevant area is the fluctuating pressure on the sheet depending on the position of the sheet and also a deformation of the sheet itself. This is caused by the growing length and area of the sheet guiding on an air cushion, and the short dwell time during the return motion when the gap between the sheet and guiding surface is open from the back, which opening causes the negative pressure to degrade.